1. Field of the Invention
The present invention relates to a method of manufacturing a vitreous silica crucible which is suitably used for pulling a silicon single crystal.
2. Description of the Related Art
A silicon single crystal has been manufactured by the Czochralski method (the CZ method) using a vitreous silica crucible. In this method, polycrystalline silicon raw material is melted and retained in the vitreous silica crucible, a seed crystal of silicon single crystal is dipped into the silicon melt, and the seed crystal is gradually pulled while rotating it, to produce a silicon single crystal by use of the seed crystal as a core.
The vitreous silica crucible used in the CZ method is manufactured by the rotating mold method where silica powder is supplied into a rotating mold to form a silica powder layer, and the silica powder layer is heated and fused by arc discharge generated by carbon electrodes. In the rotating mold method, the arc-fused portion reaches a high temperature of over 2000 deg. C.
The thus-manufactured vitreous silica crucible has two-layer structure including an outer layer containing a number of bubbles and a transparent inner layer.
It is known that the property of the crucible inner surface which contacts silicon melt while pulling a single crystal influences the property of the obtained silicon single crystal, and thus influences the yield of silicon wafers which are the final products.
Specifically, for example, when a single crystal is pulled by use of a vitreous silica crucible, melt surface vibration occurs and thus proper seeding of a seed crystal becomes difficult. In this case, there frequently occured problems that pulling of a silicon single crystal is prevented or single crystallization is prevented. The phenomenon is called melt surface vibration, and melt surface vibration is more likely to occur as the increase of the diameter of a silicon single crystal. Furthermore, it is known that melt surface vibration is related to the inner surface property of a vitreous silica crucible. JP-A-2002-154894 discloses one method of coping with this problem.
Furthermore, in order to obtain a wafer having a diameter of 300 mm or more, i.e. approx. 450 mm, it has been demanded to increase the diameter of a silicon single crystal. This demand elongates the time for pulling a single crystal, and thus elongates the time during which the crucible inner surface contacts silicon melt of 1400 deg. C. or more. This causes the following problem.
When the time for pulling is elongated, the contact time of the crucible inner surface with silicon melt is also elongated. In this case, the crucible inner surface reacts with silicon melt, to cause crystallization in the surface or a shallow layer from the surface of the crucible inner surface. The reaction causes ring-shaped brown cristobalite (hereinafter, the ring-shaped cristobalite is referred to as “brown ring”). A cristobalite layer is not formed in the inside of the brown ring, or if any, the cristobalite layer is a thin layer. The brown ring increases the area as the increase of the operation time, and the adjacent brown rings merge and grow. Finally, the center of the brown ring is corroded to expose irregular vitreous silica corroded surface.
When tiny pieces of vitreous silica detaches from the vitreous silica corroded surface, dislocation is more likely to occur in the silicon single crystal, and thus deteriorates the single crystallization yield. In particular, in order to grow a silicon single crystal for manufacturing a wafer having a diameter of 300 mm, it is necessary to continue the operation of the CZ method for 100 hours or more, and thus the vitreous silica corroded surface is more likely to appear.
It is considered that the aforementioned brown ring is generated from a core which is a tiny scratch on the vitreous silica surface, a crystalline residual portion which is an unfused portion of material silica powder, or a defect of vitreous silica structure. It is considered that the number of the brown rings can be reduced by maintaining a good surface state of vitreous silica, or reducing the crystalline residual portion by fusing silica powder at higher temperature and for a longer time in the vitreous silica crucible manufacturing process. Furthermore, as described in JP-B-2811290 and JP-B-2933404, amorphous synthetic silica powder can be used as material silica powder for forming the inner surface.